1. Field of the Invention
The invention relates to a method and device for establishing a transcoder-free operation connection (TrFO) between two communication terminals (MS) in a communication network.
2. Description of the Related Art
In order to use transcoder-free operations (TrFO), a common codec type or in the case of a multi-rate codec type a common set or common configuration of codec modes has to be negotiated between the network units (UEs, RNCs, MSCs) involved. To this end, a switching unit (MSC) has access to a storage unit containing information about the radio network controller (RNC). The storage unit for example contains an input specifying the codec modes supported by the access network (UTRAN) or the radio network controller (RNC). This storage unit is downloaded by the operator for each O&M (operation and maintenance) device and contains a list of the codec modes.
However the access network UTRAN with radio network controllers RNC only supports quite specific combinations of codec modes. It would be too complex to support all codec modes in all combinations and would take up a large number of resources. The information required to negotiate the codec mode to be used for the transcoder-free operation is exchanged between the communication terminals (MSs) and switching units (MSCs) at the Iu interface by NAS signaling or at the Nc interface by BICC signaling.
According to the content of the storage unit, a radio network controller RNC 1 for example supports the codec modes a, b, c, d, f in the combinations (codec mode configuration) a/b and b/c/d/f. The supported codec mode configurations have to be taken into account when negotiating the codec modes. Currently only individual codec modes are negotiated, not codec mode configurations, which each includes a number of codec modes. This means that it is sometimes not possible to establish a TrFO connection, as with the plurality of possible codec mode configurations (15 codec mode configurations are currently defined), it is very probable that the two radio network controllers RNCs involved in negotiating a TrFO connection support an overlapping number of identical modes but only in different codec mode configurations. If the radio network controller RNC2 supports the code mode configurations “a/b/c” and “f/g/h” for example, it is currently not possible to establish a transcoder-free operation connection between the radio network controller RNC1, which supports a/b and b/c/d/f, and the radio network controller RNC2.